A mobile communication system includes a plurality of base stations each having a service area (cell) and a mobile station moving in the service area (cell). In this type of mobile communication system, a GPS (Global Positioning system) receiver is used to achieve synchronization between the base stations. In other words, this method of synchronizing the base stations (system for synchronizing the base stations) is a method (system) in which the plurality of base stations receive UTC (Coordinated Universal Time) from the GPS and achieve synchronization according to the UTC time received by both or one of them.
In some of the base stations, the GPS receiver provided in the base station may not be able to capture any of the GPS communication satellites and thus the relevant base station may not be locked to the UTC time. This state is called “the GPS unlocked state” in this field of technology. Various time synchronizers have been proposed, which are designed to be able to achieve synchronization between base stations even in the GPS unlocked state.
For example, Japanese Unexamined Patent Application Publication No. H11-154920 (hereafter, referred to as “Patent Document 1”) discloses a clock synchronization method for a synchronous base station control system designed such that clock synchronization for the synchronous base station control system can be maintained more correctly even when a time signal cannot be received from any of GPS communication satellites. According to the clock synchronization method disclosed in Patent Document 1, if the base stations are capable of receiving a time signal from any of the GPS communication satellites, the base stations receive a GPS time signal to produce a PPS (Pulse Per Second) signal. The base stations generate a PPS clock based on this PPS signal to thereby perform clock synchronization. In contrast, if the base stations are not capable of receiving a time signal from any of the GPS communication satellites, clock synchronization is performed based on RTC (Real Time Clock) generated by the base stations. The PPS signal is generated even when a time signal cannot be received from the GPS communication satellites. The RTC is monitored whether or not it is in accord with the PPS clock, and if the RTC is not in accord with, it is corrected based on the PPS clock. A PSC (PPS Signal Counter) counting the RTCs and the PPS signals is initialized upon receiving the time signal.
FIG. 1 is a block diagram showing a configuration of the time synchronizer disclosed in the above-mentioned Patent Document 1. The illustrated time synchronizer is composed of a central processing unit (CPU) 101, a local clock generator 102, a ROM (Read Only Memory) 103, a RAM (Random Access Memory) 104, a GPS antenna 105, a GPS receiver 106, and a PPS receiver 107.
The GPS receiver 106 receives a time signal provided by a GPS communication satellite at the GPS antenna 105 to produce a PPS signal and transmits the PPS signal to the PPS receiver 107 through a line L1. The PPS receiver 107 generates a PPS clock based on the PPS signal and provides this to the CPU 101. The PPS signal is a signal generated based on a time signal which is substantially equal to the standard time signal, and is provided every one seconds. Accordingly, as long as the time signal can be received from any of the GPS communication satellites, the CPU 101 is provided with a PPS clock at accurate timing, whereby the clock synchronization can be maintained correctly.
The local clock generator 102 generates a RTC, and the RTC is monitored by the CPU 101 whether or not it is in accord with the PPS clock. If the RTC is not in accord with the PPS clock, it is corrected by a correction unit composed of the CPU 101, the ROM 103 and the RAM 104.
However, the above-mentioned related art illustrated in FIG. 1 has problems as described below.
A first problem is that correct time cannot be obtained if the base stations face a situation in which they cannot capture any of the GPS communication satellites. The reason is as follows. When the base stations come into a situation in which they cannot capture any of the GPS communication satellites, the system-side clock is corrected by seconds by means of the RTC. However, the RTC itself is a circuit designed based on an accuracy of about one second. Therefore, according to this method, if a system is configured to operate in synchronization with the UTC (Coordinated Universal Time), the system can only be corrected at an accuracy of about one second.
A second problem is that the correct accuracy can be maintained only at a level of seconds. This is because the system is designed such that correct time is obtained by correcting the RTC by comparing a PSC (PPS Signal Counter) value obtained by counting, per second, PPS signals output from the time synchronizer utilizing the GPS communication satellites with the information in seconds of the RTC clock.
Additionally, Japanese Patent No. 3,379,698 (hereafter, referred to as “Patent Document 2”) discloses an inter-base-station synchronization system in which a system clock is synchronized with the UTC time even in the GPS unlocked state. In the inter-base-station synchronization system disclosed in Patent Document 2, each of base stations comprises a GPS receiver receiving its own UTC time from any of GPS communication satellites, a clock generator generating a clock signal in synchronism with its UTC time, a computing unit computing a delay correction time according to delay time between its UTC time and another UTC time received from another base station through a transmission channel, and a memory unit for storing the traffic state of the transmission channel and the delay correction time. Each of the base stations generates a clock signal synchronized by adding the delay correction time to the above-mentioned another UTC time when it cannot capture its UTC time from any of the GPS communication satellites. The above-mentioned delay correction time is computed and output based on a difference between its own UTC time and another UTC time locked by another base station via the transmission channel when locked to its UTC time.
In the inter-base-station synchronization system disclosed in Patent Document 2, other UTC times must be periodically received from another base station via a transmission channel to compute a delay correction time.
Japanese Unexamined Patent application Publication No. 2000-332678 (hereafter, referred to as “Patent Document 3”) discloses a synchronization maintenance method according to which in a state in which synchronized time information cannot be acquired from any of GPS satellites, synchronization can be maintained with a high accuracy and a mobile station which is not connected is allowed to hold synchronized time. According to the synchronization maintenance method disclosed in Patent Document 3, in the case in which synchronized time information cannot be received from any of the GPS satellites, a phase difference between a received GPS time clock and an internally generated clock is detected, and the phase of the internally generated clock is controlled to eliminate the phase difference, so that synchronization is performed using the controlled clock.
According to the synchronization maintenance method disclosed in Patent Document 3, it is not known how the GPS time is received when the synchronized time information cannot be received from any of the GPS satellites.
Japanese Unexamined Patent Application Publication No. 2005-318196 (hereafter, referred to as “Patent Document 4”) discloses an inter-base-station synchronization system in which synchronization can be achieved among base stations even if the delay time in transmission channels among the base stations fluctuates. In the inter-base-station synchronization system disclosed in Patent Document 4, a base station to be clock synchronized with a plurality of other base stations comprises a time information receiving unit, an input unit, a delay time detection unit, a memory unit, an input clock signal monitoring unit, and a clock signal generation unit. The time information receiving unit receives time information from any of satellites to generate its own clock signal using the received time information. The input unit inputs clock signals from at least two other base stations as input clock signals. The delay time detection unit detects delay time of each of the input clock signals with respect to its own clock signal by using its own clock signal generated by the time information receiving unit and the input clock signals inputted by the input unit. The memory unit stores the delay time detected by the delay time detection unit. The clock signal monitoring unit monitors each of the input signals inputted by the input unit to detect variation in the input clock signals.
The inter-base-station synchronization system disclosed in Patent Document 4 requires constant monitoring of variation in the input clock signals.